How to make child collide
Found this answer in Mike Pan's and Dalai Felinto's marvellous book Game Development in blender (cengage learning, 2014). I will re-word it to avoid any copyright infringement.



This can be solved with the Compound option for collision bounds.
Make sure all the objects you have are collision bound enabled and then check the "Compound" box, described as "Add children to form a compound collision object". If you check this option on not only the parent, but all the children as well you get a parent object that dynamically adds the children to the parents collision bounds.



young parents, remember this as a way to control your children.